Debiting after contract was cancelled

At the end of my phone contract they are billing me an additional month, after the service comes to an end. When querying this they say they bill in arrears yet I have invoices from the start of the contract when they billed me in advanced, when the contract started. When making this evident to them they had no answer for me. When asking for a superiors email address to take things further, they give me a contact address for a business which is not affiliated with Vodacom.